Field of the Invention
The present invention relates to an image forming apparatus that forms an image using a plurality of light beams.
Description of the Related Art
In a laser scanning image forming apparatus, a plurality of main scanning lines are scanned, simultaneously using a plurality of light beams in order to raise the print speed. In addition, a high resolution is achieved by narrowing the interval of the light beams. One of the light sources that achieve this is a vertical cavity surface emitting laser (hereinafter, VCSEL). Automatic power control (APC) is employed in order to obtain a drive current with which a predetermined light quantity can be achieved with respect to the light beams. Conventionally, the APC is executed for all light-emitting portions during one scan cycle. However, as the number of light-emitting portions increases from 8 to 32, it has become difficult to execute the APC for all light-emitting portions during one scan cycle. Japanese Patent Laid-Open No. 2004-106419 proposes cyclic APC in which the light-emitting portion to be subjected to the APC is circulated by distributing the APC of a plurality of light-emitting portions to a plurality of scan cycles.
Incidentally, a standby current (which may be called bias current Ib) is caused to flow through each light-emitting portion, the standby current being for causing light-emitting portions to emit light to the extent to which a photosensitive member is not exposed, in order to improve the responsiveness of the light-emitting portions. The value of the bias current Ib is calculated based on the value of a threshold current Ith. Since this threshold current Ith varies depending on environmental conditions or the like, an appropriate value thereof needs to be found. The threshold current Ith is determined using a linear relationship (I-L characteristic) between the drive current and the light quantity, the linear relationship being obtained by measuring light quantities Pl and Pm at the time of causing a current Il and a current Im to flow through a light-emitting portion, respectively. Furthermore, a switching current Isw that is caused to flow when scanning the photosensitive member also needs to be determined for each light-emitting portion before scanning the photosensitive member. The switching current Isw is calculated by obtaining a current Ih with which a predetermined light quantity Ph is obtained and subtracting the threshold current Ith from the current Ih. Thus, three light quantities, namely the light quantities Pl, Pm, and Ph are useful in light quantity control, and the APC is executed such that the light quantities of laser beams which are incident on a light-receiving element are the three light quantities Pl, Pm, and Ph. The above APC will be called APCL, APCM, and APCH, respectively.
Since three types of APC are thus necessary, if, for example, 32 light-emitting portions exist, the APC needs to be performed 96 times. Applying the cyclic APC, for example, the APCL is executed in the first scan cycle, the APCM is executed in the second scan cycle, and the APCH is executed in the third scan cycle. Note that one scan cycle includes a period (image period) in which scanning light scans the photosensitive member and a period (non-image period) in which the scanning light scans portions other than the photosensitive member, and the APC is executed during the non-image period. For this reason, the APCH is performed during the non-image period and thereafter the image period (data mode) begins in one scan cycle, and the APCM or the APCL is executed and thereafter the image period begins in another scan cycle.
As mentioned above, Ith is calculated in the APCM and the APCL and thereafter Ih is detected in the APCH, and Isw is thereby calculated from Ih and Ib that is calculated based on Ith. For this reason, if the image period begins immediately after executing the APCM or the APCL, Ih cannot be detected since the APCH has not been performed. In this case, Isw is not fixed until image data is input, and therefore Isw is fixed by referencing Ih that has been detected at the previous time. That is to say, since the calculation of Isw is started after image data is input, transient response of the drive current occurs in accordance with the calculation time of Isw, and the waveform of the laser beams is blunted.
In an image forming apparatus, the sensitivity of the photosensitive member or the like changes dues to the environmental temperature or the like, and accordingly the density varies. In this regard, in order to maintain a fixed density by controlling the light quantity of the laser, a method for correcting the switching current Isw by multiplying it by a correction coefficient α is conceivable. However, the timing of multiplying the switching current Isw by this correction coefficient α is when image data is input in one scan cycle. This is because the switching current Isw can be accurately corrected by performing the calculation using a possible latest correction coefficient α. For this reason, immediately after starting image formation, transient response of the drive current occurs in accordance with the calculation time taken for calculating Isw×α, and the waveform of the laser beams is blunted.